As one of contributing factors toward cogging-torque generation, asymmetrical magnetic characteristics (magnetic-circuit distortion) around the center of a rotor-rotating axis can be pointed out, which is caused by a stress distribution in a stator core (hereinafter occasionally referred to as a stator iron core) with respect to the rotor-rotating direction (hereinafter occasionally referred to as simply the rotating direction). Such stress distribution in the stator core is considered to be caused by non-uniform constriction using a frame. This is mainly caused by the irregularity of the thickness of the frame around the rotor rotating axis. Generally, a rectangular frame (a frame in which the edge-shape is a rectangular pillar) tends to have mainly been used, resulting mainly in the stress generation due to the irregularity of the thickness.
Regarding a conventional rotary electric machine in which the cogging torque caused by the magnetic-circuit distortion in the stator core is focused, in a servo-motor composed of a stator having a metal frame, a rotor disposed in a space inside the stator, and a housing having the loading side and the non-loading side that rotatably support the rotor at both the ends of the rotor shaft, when the stator is produced by heat-shrink fitting in which the core is fitted in such a way that the metal frame is formed to have fins, the thickness of the frame main body at the bottoms of the fins is made approximately regular, an iron core is inserted into the metal frame after having been heated, then the metal frame is cooled, or produced by an adhesion method in which the metal frame and the iron core are fixed using a heat-curing adhesive, so that distortion given to the core and increase of the cogging torque are prevented (for example, refer to Patent Document 1).
[Patent Document 1]
Japanese Laid-Open Patent Publication 95,199/2001